THERMAL HISTORY AND TECTONIC EVOLUTION OF THE WESTERN CARPATHIANS

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1 THERMAL HISTORY AND TECTONIC EVOLUTION OF THE WESTERN CARPATHIANS Ph.D. candidate: BENEDETTA ANDREUCCI, I course Tutor: Prof. MASSIMILIANO ZATTIN Cycle: XXV Abstract The Cenozoic tectonic evolution of the Carpathians is characterized by the interaction of various geodynamic processes. In this study low temperature thermochronology is used to investigate the response of the chain at shallow crustal levels and to provide temporal and spatial constraints to better describe its evolution. AHe, AFT and ZHe dating are applied to samples belonging to the Western Carpathians, so far the less studied area of the chain. Preliminary results referred to the Polish sector of Western Carpathians show a range of exhumation ages between 6.2 and 23 Ma, post-thrusting ages (6-11 Ma) being confined to the eastern, and syn-thrusting ones (11-23) to the western area. Two exhumation process are therefore hypothesized, the first one, coeval to thrusting, appears driven by erosion of the orogenic wedge during its formation, whereas the second one, post dating thrusting, is likely to be related to the extensional tectonics which leaded to the opening of Pannonian Basin. Introduction In this study low temperature thermochronology is applied to the western sector of the Carpathians, an orogen formed between Upper Jurassic and Lower Miocene by the collision between the European Platform and the Alcapa and Tisza-Dacia microplates. Traditionally, the Carpathians have been divided into two domains: (i) the Inner Carpathians, a prolongation of the eastern Calcareous Alps, formed between the Upper Jurassic and the Upper Cretaceous; (ii) the Outer Carpathians, a classic thin skinned orogenic wedge, formed between Upper Oligocene and Lower Miocene by N-NE verging accretion. The Carpathians are still poorly known from a geological point of view due to the political history of the region and the language barriers. As a consequence, tectonic and stratigraphic units and the geological cartography are often unclear and need to be revised and updated. The aim of this work is therefore to provide new constraints for a better comprehension of the tectonic and geodynamic evolution of the Pannonian-Carpathian region. The Tertiary-Quaternary evolution of the Carpathian orogen is characterized by the interplay of different geodynamic processes including SW- to W-dipping subduction, slab retreat, and back arc extension (e.g. Doglioni et al., 2006; Nemcok et al., 2006). The response of the chain at shallow crustal levels was therefore marked by different episodes of thrusting and extension that gave rise to the construction and following disruption of the orogenic wedge. Rocks were then moved to surface through erosion or tectonic exhumation processes according to the prevailing tectonic regime. Hence thermochronological data could provide key information to unravel modes and timing of the geodynamic evolution. More in detail, this study provides the opportunity to (i) detect the sensitivity of low-t thermochronology to regional geodynamic processes and to define limitations and potentials of this method in analogous geological contexts; (ii) to obtain temporal and spatial constraints to the reconstruction of the geodynamic evolution of the chain and verify the occurrence of processes such as slab roll-back and break off. Methods Through the application of different thermochronometers to the same rock sample it is possible to reconstruct its thermal history up to the maximum temperature detectable by the applied methods; an estimate of the thermal sensitivity of a thermochronometer is given by its closure temperature T c. In this study, two main thermochronological methods will be used: - Apatite (U-Th-Sm)/He dating (AHe), which standard closure temperature is T cs =60 C (Reiners et al., 2006) 1

2 - Apatite Fission track dating (AFT), with T cs =1100 C (Reiners et al., 2006) - In some cases, also Zircon (U-Th-Sm)/He analysis (ZHe), which has T cs =175 C (Reiners et al., 2006), will be made, to extend the field of research to higher temperatures and, presumably, older events. In this work, thermochronological analysis consists of several steps: - sampling, usually carried out along transects perpendicular to the tectonic structures till the undeformed foreland, always coupled with structural survey, necessary to understand the geological context and to properly relate thermal history and tectonic evolution; - extraction, from the rock samples, of apatites and zircons to be analyzed; - selection of crystals and preparation of samples (packing or assembling of grain mounts depending weather the method is AHe-ZHe or AFT); - analysis of AHe samples through quadrupole mass spectrometer and ICP-MS; analysis of AFT samples through thermal neutron irradiation and following fission track counting and track length measurement; - data processing, aimed at (i) verifying their significance (through reproducibility and independence from analytical and compositional factors) and (ii) identifying their correlation with geological and topographical features; - data interpretation, consisting firstly in the reconstruction of exhumation history of each single sample and secondly, considering the whole dataset, in the identification of the geodynamic causes of exhumation and formulation of a geodynamic model for the area; This last step is carried out through the employment of softwares dedicated to thermal and exhumation modeling (e.g. HeFTy; Ehlers et al., 2005). Most of the samples analyzed in this study belong to sedimentary rocks: in this case the resulting thermochronological ages can assume three different meanings. A reset rock sample underwent a temperature higher than T c for a geologically significant amount of time, the mean age inferred from the thermochronological analysis of single crystals of the rock is therefore, to a first approximation, an estimate of the age of exhumation of rock sample through the closure depth. A non reset sample never suffered, after deposition, a significant heating: the single crystal ages correspond therefore to the ages of cooling of the source rocks and can be grouped in different age-populations and used to infer paleogeographical information. A partially reset sample underwent a heating process insufficient to completely reset the thermochronometer, due to its too low temperature and/or short holding time, therefore neither mean age or single grain ages are geologically significant. Activities of the first year The first year started with processing of already existing samples. So far, 39 AHe and 18 ZHe ages have been obtained. 20 AFT samples are ready to be analyzed in the months of November- December The AHe and ZHe analyses have been carried out in the months of February-April 2010 at the Arizona Radiogenic Uranium Dating Laboratory (Department of Geosciences, University of Arizona, Tucson) under the supervision of prof. Peter Reiners and doc. Stefan Nicolescu. A sampling campaign associated to structural survey was performed along the Ukrainian Carpathians: 22 new samples were collected and, after processing, they will be analyzed during the first months of the second year of PhD. Finally, a preliminary thermal modeling (based on the HeFTy software) has been made on some selected AHe and AFT samples. Preliminary results The results achieved so far are strictly referred to the Outer Polish Carpathians. The reset samples of the western sector show scarcely homogeneous Lower-Middle Miocene AHe and AFT ages, hence coeval to thrusting (18-12 Ma, Sperner et al., 2002); the time-gap between 2

AHe and AFT ages spans between 2 and 15 Ma. The reset samples of the eastern sector show, instead, very homogeneous Late Miocene AHe and AFT ages (6.2-11.2 Ma), with a AHe-AFT ages time-gap of 0-2 Ma.

The first phase, in the Central-Western region dating back to Early-Middle Miocene can be described as syn-thrusting exhumation due to erosion of the orogenic wedge during its formation.

3 AHe and AFT ages spans between 2 and 15 Ma. The reset samples of the eastern sector show, instead, very homogeneous Late Miocene AHe and AFT ages ( Ma), with a AHe-AFT ages time-gap of 0-2 Ma. Two exhumation phases are then hypothesized. The first phase, in the Central-Western region dating back to Early-Middle Miocene can be described as syn-thrusting exhumation due to erosion of the orogenic wedge during its formation. The second phase, post dating thrusting, appears driven by extensional tectonic processes, as pointed out by the well documented presence of normal and detachment faults dissecting and reactivating thrusts (Mazzoli et al., 2010). This second phase matches well with the timing of the Late Sarmatian-Early Pannonian extensional phase. The eastward younging trend of thermochronological data is compatible with subduction retreat (Nemcok et al., 2006) but also with slab-break off, proposed by some authors (e.g. Nemcok et al., 1998), suggesting the existence of some relationships between exhumation and evolution of the Pannonian basin (Nemcok et al., 2006). PL 22 PL 10 a) b) Figure 1: Thermal histories of selected samples and related tectonic models. The thermal histories were modeled through the software HeFTy (Ehlers et al., 2005); the black lines represent the thermal histories which best fit the empirical data; the orange lines represent the constraints on the thermal history. In (a) the modeled sample is PL 22, belonging to the western sector ( AFT age measured: / -3.8, AFT age model: 23.3, GOF: AHe age measured: 16.0 ±0.5, AHe age model: 16.0, GOF: Contraint: T>120 C between 35 and 25 Myr); the tectonic model shows the hypothesized exhumation process for PL 22 and other Early-Middle Miocene samples. In (b) the modeled sample is PL 10, belonging to the eastern sector (AFT age measured: / -2.6, AFT age model: 11.2, GOF: 0.86; AHe age measured: 6.24 ±0.3, AHe age model: 6.26, GOF: 0.96; Constraint: T>120 C between 35 and 25 Myr ). The tectonic model shows the hypothesized exhumation process for PL 10 and other Late Miocene samples. 3

4 References DOGLIONI, C., CARMINATI, E. AND CUFFARO, M Simple kinematics of subduction zones. Intern. Geol.Rev., 48, EHLERS, T.A., CHAUDHRI, T., KUMAR, S., FULLER, C.W., WILLETT, S.D., KETCHAM, R.A., BRANDON, M.T., BELTON, D.X., KOHN, B.P., GLEADOW, A.J.W., DUNAI, T.J. AND FU, F.Q., Computational tools for low-temperature thermochronometer interpretation. Rev. Min. Geoch., 58, MAZZOLI, S., JANKOWSKI, L., SZANIAWSKI, R. AND ZATTIN, M., Low-T thermochronometric evidence for post-thrusting (<11 Ma) exhumation in the Western Outer Carpathians, Poland. Compte Rendue Geosci., 342, NEMČOK, M., POSPÍŠIL, L., LEXA, J., DONELICK, R.A., Tertiary subduction and slab break-off model of the Carpathian Pannonian region. Tectonophysics, 295, NEMČOK, M., POGACSAS, G., POSPÍŠIL, L., Activity timing of the main tectonic systems in the Carpathian pannonian region in relation to the rollback destruction of the lithosphere. Am. Assoc. Pet. Geol. Mem 84, REINERS, P.W. AND BRANDON M.T., Using thermochronology to understand orogenic erosion. Annu. Rev. Earth Planet. Sci., 34, ROYDEN, L.H., Late Cenozoic Tectonics of the Pannonian Basin System. In: The Pannonian Basin, astudy in basin evolution (Royden, L.H. and Horváth, F., eds.), Am. Assoc. Pet. Geol. Mem., 45, ROYDEN, L.H. AND HORVÁTH, F., The Pannonian Basin, a study in basin evolution, Am. Assoc. Pet. Geol. Mem., 45, 375 pp. SPERNER, B., RATSCHBACHER, L. AND NEMČOK, M., Interplay between subduction retreat and lateral extrusion: tectonics of the Western Carpathians. Tectonics, 21,

5 SUMMARY LAST YEAR S ACTIVITY Courses: M.FLORIS: Introduzione alle tecniche GIS, Dipartimento di Geoscienze, Università degli Studi di Padova. E. CALANDRUCCIO: Corso di inglese parlato, Dipartimento di Geoscienze, Università degli Studi di Padova. L. GULIK: Corso avanzato di inglese scientifico, Dipartimento di Geoscienze, Università degli Studi di Padova. G. ARTIOLI, G. DI TORO, A. FIORETTI: Corso di comunicazione scientifica, Dipartimento di Geoscienze, Università degli Studi di Padova. P. REINERS: Geochronology and Thermochronology, Department of Geosciences, University of Arizona, Tucson (AZ). Communications: ANDREUCCI, B., JANKOWSKI, L., MAZZOLI, S., SZANIAWSKI, R., ZATTIN, M An example of two stages exhumation of a thrust and fold belt from the thermochronology of Western Carpathians. Congresso della Società Geologica Italiana. Pisa, 6-8 settembre Posters: ANDREUCCI, B., JANKOWSKI, L., MAZZOLI, S., SZANIAWSKI, R., ZATTIN, M Two stages Neogene Exhumation of Western Carpathians. Thermo 2010 congress September 2010, Glagow (UK). Publications: ZATTIN, M., ANDREUCCI, B., MAZZOLI, S., JANKOWSKI, L., SZANIAWSKI, R., Two stages Neogene Exhumation of Western Carpathians. Submitted to Terra Nova. Stages in other universities February-April 2010: laboratory activity at the Arizona Radiogenic Helium Dating Laboratory, Department of geosciences of University of Arizona, Tucson (AZ). under the supervision of prof. Peter Reiners and doc. Stefan Nicolescu. Field work June 2010: sampling campaign and structural survey in the Ukrainian Carpathians. 5

TECTONOTHERMAL EVOLUTION OF THE CENTRAL-WESTERN CARPATHIANS AND THEIR FORELAND

TECTONOTHERMAL EVOLUTION OF THE CENTRAL-WESTERN CARPATHIANS AND THEIR FORELAND Ph.D. candidate: ADA CASTELLUCCIO, III course Tutor: Prof. MASSIMILIANO ZATTIN Co-tutor: Prof. STEFANO MAZZOLI Cycle: XXVII